this is a lazy, clumsy editing attempt, done through a document registration service which exists to prevent exactly this and yet, you have to be an experienced nerd (eg https://en.wikipedia.org/wiki/Matthew_Garrett has a doctorate and decades of software development experience) who will jump through a bunch of hoops to even begin to build a case beyond he-said-she-said. And he still doesn't have a settlement or criminal conviction in hand, so he's not even half done... Or look at the extensive forensics in the Craig Wright case just to establish simple things like that they were edited or backdated to a legally acceptable level.
Meanwhile, the original PDF edit in question took maybe 5 minutes with entry-level PDF tools.
I think Gwern was too focused on academic PDFs posted publicly to support some argument. There, it's actually advantageous to make a completely new PDF with your faked data, since it prevents people from finding the original and doing a comparison.
But for official documents used as legal proof, staying as close as possible to the original is required for a forgery to avoid sticking out simply for having the wrong formatting. But you usually won't find those posted publicly on the internet.
